1. Field of the Invention
The present invention relates to a method of finding a driving strength, and particularly to a method of finding an output driving strength of a target cell, such as an intellectual property cell, from a mixed-signal integrated circuit, and a record medium accessible by a computer program to execute the method thereof.
2. Description of the Related Art
Due to the trend of high density of integrated circuits and time-to-market products, how to develop a cost-effective integrated circuit in a shortest time becomes an essential topic. Design reuse is one approach to solve the issue. In other words, various designed/laid-out circuits are reused and assembled by application specific integrated circuits (ASIC). Accordingly, development of chip products can be accomplished in a shortest time. The aforesaid repeatedly used designed/laid-out circuit is called an intellectual property (IP) cell.
Before using IP cells to design integrated circuits, the output driving strength of the cells are needed by the IC designers. Generally, driving strengths of output pins of a cell are represented by maximum load capacitances. Cell attributes including the output driving strengths of standard cells (maximum load capacitances), are recorded in a standard cell library. Normally, the best, the typical, and the worst driving strengths of each standard cell can be found in the best, the typical, and the worst standard cell libraries, respectively.
For cells not included in the standard cell library, a SPICE model or other simulation software must be used to analyze the target cell for obtaining the driving strengths of the output pins, including the bi-directional pins, of the target cell. If the target cell which is going to be simulated is very complicated, the required time for simulation will exponentially and dramatically increase. For example, the HSPICE software provided by Synopsys Co. simulates the timing delay of the phase locked loop (PLL) may take 4-5 days and Analog-to-Digital Converter (ADC) may take 3+ weeks. Moreover, in order to find the driving strengths of the cell, the SPICE needs executing more steps and will be very time consuming, such as several months. The length of execution time grows exponentially is called Non-Polynimial (NP) problem. Accordingly, the use of the SPICE simulation software to analyze the driving strengths of the target cell, such as the IP cell, wastes a lot of time and becomes impractical.